The present invention relates in general to time-to-intercept determination and more particularly to a system and a method for accurately determining a closing time between a radiation source and an object by passively sensing the irradiance of the radiation source.
Time-to-intercept determination has several important civilian and military applications. Time-to-intercept (TTI) is the amount of time remaining before an object makes contact with (xe2x80x9cinterceptsxe2x80x9d) another object. Determination of the TTI is used in civilian applications such as, for example, vehicular motion measurements, factory measurements, astronomical measurements, satellite measurements and aircraft collision avoidance systems. One military and civilian use for TTI determination is in missile warning systems (MWS). In this age of increased conflict and terrorism, both civilian and military aircraft are especially vulnerable to missile attack, particularly from infrared guided missiles. In fact, over ninety percent of military aircraft losses worldwide since 1980 have been the result of infrared guided missile attacks, and there also have been attacks on civilian aircraft. Due to the lack of missile warning, most pilots of the downed aircraft were not aware of the missile firing until the actual hit occurred, thus could not initiate evasive or protective actions. A MWS carried by an aircraft also enhances the effectiveness and the available duration of aircraft self-protection such as flares, which is most effective when used a certain time before the missile intercepts the aircraft. Therefore, in order to be useful and to ensure maximum self-protection and facilitate use of available self-protection resources, the MWS must have an accurate TTI determination.
In both civilian and military applications it is important that the TTI be determined as accurately as possible to avoid possible disaster. For example, an error in the TTI determination may result in lack of effectiveness for a self-protect system, if a flare is dispensed too late or too early. This may mean the loss of an aircraft and human lives. Thus, there exists a need for an accurate, effective and low-cost system and method for TTI determination.
One way to determine the TTI is by using a passive sensor system. This type of system is generally preferred over other types of systems because of the disadvantages associated with the other systems. For example, an active sensor system detects range by measuring pulse-doppler returns. The active sensor system, however, is relatively complex, heavy and costly because of the rangefinding equipment that must be used. Similarly, TTI determination using a triangulation system is costly, slow and requires two sensors with the same field of regard placed far enough apart to achieve the high angular accuracy.
Unlike an active sensor system that employs radar, lidar or other direct range-measuring equipment, a passive sensor system cannot directly measure the TTI and must indirectly measure the TTI. However, TTI can be determined from fundamental physical laws, if the object has a uniform radiation emission (such as thermal self-emissions, electromagnetic transmitting or reflected power). These radiation emissions may be from natural sources (such as the sun) or from artificial sources (such as laser, radar and searchlights).
For example, a passive ultraviolet sensor system indirectly measures the TTI by observing photon scattering effects that are inherent in these short ultraviolet wavelengths. As two objects approach each other this causes a noticeable xe2x80x9cdiffusionxe2x80x9d. The main disadvantage of this passive ultraviolet sensor system, however, is that it is not useful in the infrared spectrum where this scattering is virtually nonexistent. Moreover, the detection range in the ultraviolet spectrum is quite short due to the limited transmittance of the atmosphere in this spectral region.
Another type of passive sensor system indirectly measures the TTI by sensing the irradiance associated with an object. In general, this type of system measures the irradiance of the object at two instances in time and computes the TTI from an equation relating irradiance and time. This type of system can use infrared detection, which is advantageous for long-range detection (for example, at the time a missile is launched). Long-range detection is a useful feature of a TTI determination system because it gives the other object (such as a target aircraft) time to use countermeasures (such as evasive maneuvers and infrared jamming devices).
A disadvantage of this type of prior-art system is their succeptability to noise, since the initial irradiance signal measurement is measured at a long range (when the two objects are some distance away from each other, and the signal is weak) is used throughout the computation of the TTI, making the calculations inaccurate. In fact, a major deficiency of prior-art irradiance-based passive sensor systems is that they either completely ignore or inadequately compensate for the adverse effects of noise. Noise can cause significant accuracy problems in the TTI determination.
Therefore, what are needed are a system and a method for TTI determination using an irradiance-based passive sensor system that recognizes, addresses, and adequately compensates for the adverse effects of noise, thereby increasing the accuracy and efficiency of the TTI determination.
To overcome the limitations in the prior art as described above and other limitations that will become apparent upon reading and understanding the present specification, the present invention includes a method and a system for accurately determining a closing time (or time-to-intercept) between a radiation source and an object. The present invention utilizes a plurality of features to reduce the noise present in the time-to-intercept computation and thus improve the accuracy of the time-to-intercept determination while still remaining relatively inexpensive and simple.
In a preferred embodiment, the invention includes a method for time-to-intercept determination that includes computing the time-to-intercept using at least three irradiance values of a radiation source and reducing noise in this computation by employing a variety of measures. For example, noise present in the irradiance data is monitored to eliminate the use of any excessively noisy data in the computation of the time-to-intercept. Preferably this is accomplished by computing a signal-to-noise ratio value for each irradiance value and comparing this value to a threshold signal-to-noise ratio value. Any signal-to-noise ratio values that do not meet this threshold criteria are rejected; only those irradiance values within an acceptable noise tolerance (threshold) are used in the time-to-intercept computation. This noise reduction helps improve the accuracy of the time-to-intercept calculation.
The method also includes an averaging feature that further enhances accuracy. For example, a minimum time interval is defined by the resolution of the computing device, to prevent singularities and to minimize sample-data noise in the time-to-intercept computation. The time interval between irradiance values having acceptable noise levels is determined. If this time interval is less than the minimum time interval this irradiance data is not directly used in the time-to-intercept computation. Instead, the irradiance data is averaged over the time interval thus providing additional noise suppression and improved accuracy in the time-to-intercept computation.
The time-to-intercept computation is constantly updated by using irradiance data that is advanced in time. This updating means that the previous time-to-intercept computation is replaced with an updated time-to-intercept computation that was computed using irradiance data taken later in time. Since interception is preceeded by closing distance, and since signal strength naturally increases significantly as distance decreases, this updating significantly enhances signal-to-noise ratio, which improves TTI accuracy. without updating, as in prior art schemes, computations are based on initial observations made when the radiation source (such as a missile) is first detected at long range and the signal is just at the acceptable level, hence the noise in the irradiance data is typically still high and the accuracy of the time-to-intercept computed using this data is accordingly low. By constantly updating the computation using irradiance data of the missile as it comes closer, the accuracy of computation is markedly improved.
The time-to-intercept is computed using equations that relate the irradiance values to the time-to-intercept. In addition, the computation uses signal-strength normalization in order to remove the effects of radiation source size (or absolute signal strength). Normalization is based upon assumptions of whether the closing rate follows a constant acceleration or a constant velocity trajectory. Equations for both constant acceleration and constant velocity assumptions have been developed.
The method can signal a warning system when a predetermined time-to-intercept is reached, which takes full advantage of the aforesaid updating process. The likelihood of false trigging, another aspect of TTI accuracy, is reduced, because the accuracy of the time-to-intercept determination is improved by the present invention. It is desirable that the warning system be triggered at a specific time-to-intercept, so that it can cue a prearranged response. For example, if a missile is approaching an aircraft and the time-to-intercept reaches a predetermined time, the warning system can cue the aircraft to release flares or other engage other protective anti-missile techniques at a time of optimum effectiveness. By releasing flares only when they are effective and necessary, limited countermeasure resources can be conserved so the usefulness of the countermeasure system can be extended.
Other features of the invention also improve the accuracy of the invention. For example, filtering of the irradiance data prior to use by the invention helps improve the accuracy of the incoming data and eliminate any extraneous signals. In addition, this filtering can be included in the detection system and can be a continuous filtering. This invention works with irradiance data that is provided as a continuous data stream, as a periodically-sampled data set, or as an aperiodically-sampled data set.
The method of the present invention can be implemented in a system for time-to-intercept determination. This system includes a detection system for providing irradiance data, a timing device for providing timing data, a time-to-intercept processor for providing time-to-intercept data, and a warning system for triggering countermeasures and warning the pilot of impending danger.
The time-to-intercept processor of the present invention includes an input module for receiving and filtering data, a noise threshold module for further reducing noise in the data, and an averaging module for averaging the data over a time interval. In addition, the processor includes a calculation module for calculating the time-to-intercept, and an update module for constantly updating the time-to-intercept using irradiance data that is advanced in time together with clock data to interpolate time-to-intercept between the times of irradiance measurements.
Other aspects and advantages of the present invention as well as a more complete understanding thereof will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. Moreover, it is intended that the scope of the invention be limited by the claims and not the preceding summary or the following detailed description.